Future wireless receivers will need to be able to receive signals from multiple sources simultaneously. For example, in automotive multi-media receivers, it is highly desirable to receive FM radio signals with embedded traffic information together with Digital Audio Broadcast (DAB) signals with superior audio quality.
Because of the need for concurrent reception, the analog portion of the receiver chain needs to be duplicated for each of the simultaneously received signals. The individual receiver chains can be optimized for a specific wireless communication standard.
In between the analog portion of the receiver chain and the digital portion of the receiver chains, sits a so-called Analog-to-Digital Converter (ADC). This ADC translates the received signal into a digital representation for further processing. If the entire receiver consists of multiple chips, then this ADC is most often integrated with the digital portion of the receiver. This is also the case when the receiver is integrated as a single chip.
For concurrent reception, it would be desirable to share this ADC between the multiple receiver paths, and to digitally correct any of the artifacts introduced by this sharing. The area occupied by digital circuits for a specific functionality keeps decreasing thanks to constant and rapid progress in semiconductor manufacturing. In contrast to digital circuits, which are significantly benefited by this progress, analog circuits realize little or no benefit. As this manufacturing progress continuous, the incentive grows to replace analog circuits by digital circuits where possible.
When an ADC is shared between multiple receiver paths, it can be necessary to take samples from the different signals at different rates. This subjects at least one of the signals to periodic non-uniform sampling, because the ADC needs a constant time to convert a single sample.
To illustrate this statement, consider the case where the first of two signals needs to be sampled on average three times faster then the second signal. Referring to FIG. 1, the ADC takes three samples from the first signal and then one sample from the second signal. Each of these samples is processed by the ADC in the same amount of time. The samples of the second signal are evenly spaced. The samples of the first signal, however, are non-uniform in time, but they are uniform in a periodic fashion: after every group of 3 samples, the sample period changes briefly to twice the regular sample period.
This non-uniform sampling introduces artifacts into the signal, and as such, the sharing idea cannot be used without impractical constraints on the signal. This is especially true for the reception of wireless signals, where weak wanted signals are adjacent to strong unwanted signals. Non-uniform sampling introduces aliasing and causes the unwanted adjacent signals to fold onto the weak wanted signal.
Currently, techniques exist to remove the uniform sampling artifacts (e.g., Marziliano, P. and M. Vetterli (2000). Fast reconstruction in periodic nonuniform sampling of discrete-time band-limited signals. Proceedings of 2000 International Conference on Acoustics. Speech and Signal Processing, Istanbul, Turkey, IEEE).
However, these techniques have been developed for use on a general purpose computer in the form of software, and cannot be implemented efficiently using dedicated digital circuitry. The existing techniques use matrix operations and thus have substantial memory and processing time requirements. Only if the signal reconstruction can be implemented efficiently using digital circuitry, is it economical to replace one or more ADC's by a single ADC with digital reconstruction circuitry.